Molar distalization with a partially integrated mini-implant to … · 2017-12-04 · Molar distalization with a partially integrated mini-implant to correct unilateral Class II malocclusion

CLINICIAN’S CORNER

Molar distalization with a partially integratedmini-implant to correct unilateral Class IImalocclusion

Kyu-Rhim Chung,a Seong-Hun Kim,b Michael P. Chaffee,c and Gerald Nelsond

Seoul, Korea, Coeur d’Alene, Idaho, and San Francisco, Calif

This article illustrates a new treatment system combining segmented wire and osseointegrated mini-implantsfor molar distalization without complex appliances. The procedures, advantages, efficacy, and indications forthis method are discussed. Two patients whose treatment plans included distal molar movement andorthodontic mini-implant treatment were recruited. One patient required 1 molar to be uprighted, and the otherneeded molar distalization to regain space lost for the missing maxillary right second premolar. C-implants(diameter, 1.8 mm; length, 8.5 mm) were placed and, after 4 weeks of healing, were used as direct anchorageand indirect anchorage simultaneously for correcting the asymmetric Class II molar relationship. Few ortho-dontic attachments were necessary, and the teeth moved rapidly to the planned positions without detrimentaleffects on the occlusion. The combination of segmented archwires, minimum bonded attachments, and a par-tially osteointegrated mini-implant (C-implant) was a simple and effective treatment choice in distalizationtreatment. (Am J Orthod Dentofacial Orthop 2010;138:810-9)

Unilateral full-step Class II correction, withasymmetry in the maxillary arch, can posea challenge for the orthodontist. Although vari-

ous treatment modalities have been developed and usedsuccessfully over the years, many need intensive coop-eration from the patient. Noncompliant mechanics canbe complicated and cumbersome. Unilateral premolarextraction is usually an available treatment optionbut can result in arch skewing or displacement of themidline.

Headgears can be adjusted to provide a distalizationforce on the Class II side.1-3 Removable appliancesdesigned to distalize molars have been advocated, butboth approaches require much patient cooperation.4,5

Fixed functional devices can provide a distalizationforce to the maxillary posterior teeth but also influencethe mandibular dentition.6,7 Pendulum appliances and

aPresident, Korean Society of Speedy Orthodontics, Seoul, Korea.bAssistant professor, Department of Orthodontics, School of Dentistry, Kyung

Hee University, Seoul, Korea.cPrivate practice, Coeur d’Alene, Idaho.dClinical professor, Division of Orthodontics, University of California at San

Francisco.

The authors report no commercial, financial, or proprietary interest in the

products or companies described in this article.

Partly supported by the Korean Society of Speedy Orthodontics.

Reprint requests to: Seong-Hun Kim, Department of Orthodontics, School of

Dentistry, Kyung Hee University #1 Hoegi-dong, Dongdaemun-gu, Seoul 130-

701, South Korea; e-mail, [email protected].

Submitted, February 2008; revised and accepted, July 2008.

0889-5406/$36.00

Copyright � 2010 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2008.07.027

810

distal jets have been advocated and proven successful formolar distalization. However, there are disadvantages,including laboratory time and expense. Whereas theseappliances incorporate design components to attemptto prevent anchorage loss, flaring of the anterior teethand increased overjet usually take place to a significantextent. One negative sequela usually seen with theseappliances as posterior teeth distalize is increasedlower facial height because of clockwise mandibularautorotation.8-11

In more recent years, treatment mechanics with skel-etal anchorage combined with pendulum springs havebeen devised primarily for molar distalization. However,these biomechanical systems are somewhat complexand require significant laboratory time and expense.The distalization process is also delayed because of thewaiting period for these palatal implants to achieveosseointegration.12-15 Sugawara et al16 demonstratedthe distalization capability of a skeletal anchoragesystem involving titanium anchor plates. Althougheffective, the surgical placement of a miniplate is moredifficult and invasive than that of a mini-implant.Park17 showed the possibility of full-arch distalizationusing mini-implants. However, these mini-implantsacted as auxiliaries for continuous archwire systemsduring the whole orthodontic treatment.

We present 2 patients who had limited treatment in-corporating unilateral molar distalization with skeletalanchorage. The skeletal anchorage was provided bya C-implant, introduced as a partially integrated mini-

mailto:[email protected]

Fig 1. C-implant dependent pushing mechanics: A, schematic illustration of C-implant; B and C,0.017 3 0.025-in stainless steel archwire and open-coil spring.

American Journal of Orthodontics and Dentofacial Orthopedics Chung et al 811Volume 138, Number 6

implant system (Cimplant, Seoul, Korea). These rela-tively simple treatment mechanics use direct or indirectanchorage to achieve efficient, bodily molar distaliza-tion, while minimizing undesirable vertical dimensionchanges.

The biomechanics of molar distalization with push-ing mechanics use partially osseointegrated C-implants.A C-implant is composed of titanium grade Valloy; it isself-tapping and threaded.18-25 Each C-implant has 2components: the screw part has a diamter of 1.8 mm,and the head part measures 2.5 mm in diameter. Thehead is inserted into the screw and tapped into place.Precise fit and friction retain the head duringtreatment. The screw surface is sandblasted large gritand acid-etched treated for optimal osseointegration ex-cept for the upper 2 mm that is in contact with soft tis-sues. Upper and lower structures are mechanicallyinteroriented; this means that they can have many vari-ations. Especially, the head part can have differentlengths and different numbers of holes. According tothe treatment objectives, the head part can be changedin midtreatment, without the need to remove the im-planted screw. The 0.8-mm diameter hole of the headpart can receive archwires (Fig 1, A).

C-implants can provide absolute anchorage with theadvantage of resistance to rotation force and high stabil-ity during force application.23-25 Direct anchorage canbe obtained by using a segmental archwire with anopen-coil spring to push the teeth independently(Fig 1, B and C). Intrusive step bends are usuallymade to the segmented archwire to minimize the extru-sion tendency during distalization. Indirect anchorage

can also be established by attaching a wire ligaturewire from the C-implant to a bracket anterior to themini-implant. Mesial movement of anterior teeth canthus be prevented.

CASE REPORTS

Patient 1

A man, aged 42 years 7 months, had a chief com-plaint of loss of the maxillary right first molar and me-sial inclination of the second molar. This conditionfollowed the extraction 2 years previously of the firstmolar after a vertical fracture (Figs 2 and 3, A-C).Pretreatment intraoral photographs showed bilateralClass I canine relationships, and a Class I molarrelationship on the left side. There was a full-step ClassII second molar relationship on the right side because ofmesial drifting after the first molar extraction. Therewas a slight anterior open-bite tendency. The maxillaryand mandibular midlines were coincident.

A panoramic radiograph at the initial examinationshowed mesial drifting and tipping of the maxillaryright second molar (Fig 3, D). An implant fixture hadbeen placed into the missing molar position 2 monthspreviously. The implant was close to the second premo-lar due to inadequate space for placement at the time ofsurgery (Fig 3, D). The missing maxillary right firstmolar allowed mesial inclination of the maxillary rightsecond molar into a full-step Class II second molarrelationship. The treatment plan was to place a partiallyosseointegrated mini-implant that could endureheavy and dynamic forces in the interradicular space

Fig 3. Pretreatment panoramic radiographs of the firstpatient: A, vertical fracture was observed on the maxil-lary right first molar; B, after extraction; C, 2 years afterextraction without space maintaining; D, dental fixturewas implanted in the extraction space.

Fig 2. Pretreatment intraoral photographs of the first patient.

812 Chung et al American Journal of Orthodontics and Dentofacial Orthopedics

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between the maxillary right first and second premolars.A segmental archwire with an open-coil spring wasplanned to move the second molar distally into a ClassI second molar relationship.

A C-implant (diameter, 1.8 mm; length, 8.5 mm)was implanted in the interradicular space between themaxillary first and second premolars with a 45� apicalangulation to the long axis of the teeth (Fig 4). The an-gular placement of the mini-implant allowed for less

chance of root contact because of greater interradicularspace in the apical areas of the roots26 (Fig 5).

The segmental stainless steel wire (0.017 3 0.025in) was placed into the hole of the C-implant head tothe archwire slot of the second molar, and light ortho-dontic force (50 g) was applied with an open-coil springimmediately because that maxillary second molarshowed fast mesial tipping tendency in a short period(Fig 6, A). One month later, a crimpable hook wasplaced on the archwire to activate the open-coil spring,and intrusive step bends were made to the segmentalarchwire to prevent the extrusion tendency of the secondmolar (Figs 6, B, and 7, A). The patient was examinedevery 4 weeks to monitor progress (Figs 6, C and D,and 7, B and C). Selective grinding was done on thesecond molar to eliminate premature contacts as thetooth distalized. Two months later, the second surgeryfor the restorative implant was done. Four monthslater, distal movement was stopped, and the prosthesiswas delivered.

The final photographs and the panoramic radiographshow that adequate uprighting of the second molarprovided sufficient space for an implant crown withnormal dimensions (Figs 7, D, 8, and 9). The patient’spreexisting occlusion was maintained. An anterioropen-bite tendency remained, but a minimal increasein the anterior open bite was noted during treatment.In this minor tooth-movement case, bracketing waslimited to the maxillary right first molar only (Fig 10).The total treatment time was 3 months, after which

Fig 4. Placement procedures of C-implant in the first patient: A-C, screw part placement after pilotdrilling; D-F, head part adaptation with head tapper.


the implant prosthesis was delivered in the regainedspace (Fig 10). The C-implant was also stable duringthe total treatment and successfully removed by usingtopical anesthesia.

Fig 5. Postplacement panoramic radiograph of the firstpatient.

Patient 2

A woman, aged 22 years 7 months, visited for an or-thodontic consultation. Her chief complaints werecrowding and jaw pain. She had a Class II Division 1subdivision left malocclusion. A Class I molar and ca-nine relationship was noted on the left side. On the rightside was a full-step Class II molar relationship. Theright canine was in Class III due to the maxillary mid-line drift to the right. The maxillary right second premo-lar was extracted because it was a blockout tooth,allowing mesial drifting of the first and second molars(Fig 11). The mandibular midline was coincident withthe facial midline. The maxillary midline was to theright 2 mm, because of the previous second premolarextraction. Mesial inclinations of the maxillary rightfirst, second, and third molars were noted in the pretreat-ment panoramic radiograph. There were 2 treatment op-tions: to maintain the asymmetric key relationship onlyby conventional treatment mechanics, and to recapturethe premolar space to achieve a Class I symmetric keyrelationship. The patient requested the space-regainingtreatment for the extracted premolars without changingthe midline condition.

Therefore, the treatment goals involved recapturingthe space for the missing premolar and treating the patientwithout extractions except the lower impacted third mo-lars. Skeletal anchorage would allow distalization of themaxillary right molars. After physical therapy and 2months of stabilizing splint therapy, the temporomandib-ular joint symptoms disappeared.

The mandibular third molars and the maxillary rightthird molar were extracted. To improve the molar inter-cuspation and reestablish a Class I occlusion on the rightside, a C-implant (diameter, 1.8 mm; length, 8.5 mm)was placed in the interradicular space between themaxillary right canine and the first premolar, witha 75� apical angulation to the long tooth axis of the teethbecause of sufficient interradicular space. A 0.017 3

Fig 6. Treatment progress intraoral photographs of the first patient: A, 1 week after immediate forceapplication; B, 1 month after force application; C, 2 months after force application, with the secondsurgery performed for the dental implant; D, 3 months after force application, with sufficient upright-ing and distalization without significant root resorption and vertical dimension change.

Fig 7. Treatment progress occlusal photographs of the first patient: A, 1 month after force applica-tion; B, 2 months after force application, with the second surgery performed for the dental implant;C, 3 months after force application; D, prosthesis was applied on the implant fixture.


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0.025-in stainless steel segmented archwire was placedthat included the maxillary right canine to the right sec-ond molar (Fig 12, A). Indirect anchorage was providedby placing a ligature wire from the C-implant to the ca-nine bracket, and the distalization process was begunwith an open-coil spring between the first and second

molars. At a subsequent appointment, direct anchoragewas added with a second segmental 0.016 3 0.022-instainless steel archwire from the C-implant to the firstmolar utility arch slot, with the open-coil spring deliver-ing the 100 g of distal force (Fig 12, B, C, and G). Theremaining full maxillary arch bonding was done after

Fig 8. Posttreatment intraoral photographs of the first patient.

Fig 9. Posttreatment panoramic radiograph of the firstpatient.

Fig 10. Periapical radiographs of the first patient: A, pre-treatment; B, after molar distalization; C, after prosthesison the implant fixture; D, 1 year after treatment.


a Class I molar relationship was established (Fig 12, D,E, and H). Distal movement of the second molar was ob-served, and excessive mesial movement of the caninewas prevented by the indirect anchorage from the liga-ture wire. Distalization of the first and second molarswas accomplished by using full and segmental arch-wires, along with the open-coil spring to provide a dis-talizing force (Fig 12, E). To provide further anchorage,a transpalatal stabilization wire was placed on the rightand left first premolars (Fig 13).

The final photographs and radiographs show ade-quate molar distalization, with sufficient space forimplant placement and restoration of the second premo-lar (Fig 14). Class I molar and canine relationships werereestablished on the right side. The maxillary and man-dibular midlines were coincident. Retention was

accomplished with maxillary and mandibular fixedretainers. The final panoramic radiograph showed noevidence of root resorption of the maxillary anteriorteeth. Although greater mesial root tipping of themaxillary right first premolar would have been ideal,sufficient mesiodistal space was recaptured to makerestoration of the second premolar possible (Fig 14).The total treatment time was 12 months; then an implantand a crown were placed. The C-implant was stable dur-ing the total treatment period and successfully removedwith topical anesthesia.

Fig 11. Pretreatment intraoral photographs and panoramic radiograph of the second patient.


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DISCUSSION

These 2 patients both had indirect and direct im-plant anchorage to distalize and upright the molars.Without implant anchorage, appliance complexity andbiomechanic demands would have been much greater.Implant anchorage with a partially osseointegratedmini-implant (C-implant) permited the use of few or-thodontic attachments. Others have suggested similartactics.

Park et al27 showed a new molar uprighting methodwith miniscrew anchorage in the retromolar area. Leeet al28 suggested direct miniscrew anchorage withoutthe assistance of anchorage teeth for uprighting themandibular second molars. They mentioned that directapplication of force from the mini-impant to the targettooth eliminates the possibility of unwanted movementof the anchorage unit that can occur even with indirectminiscrew anchorage as a result of technical errors inpassive bracket placement or a weak attachment be-tween the miniscrew and the anchor tooth. Whereasthey advocated the principle of minimum usage ofa conventional mini-implant for the maximum upright-ing effect, they also showed the application of multiplemini-implants for uprighting 1 tooth.

In the clinical report of Derton et al,29 severalminiscrews were used for 3-dimensional movement oftipped molars. Gracco et al30 also suggested the innova-tive uprighter jet for preventing molar extrusion duringthe uprighting procedure. But complex distalization ap-pliances were needed, and the mini-implant waspositioned in an edentulous area.

We suggest using a partially osseointegrated mini-implant because it allows easier and more accurate ap-plication of force and vector, and even a rotational forcewithout dislodging the implant.31 A partially osseointe-grated mini-implant can be placed toward the front ofthe mouth with support lever arms that transfer forceand vector distally. Similar to a molar tube, the holeof the C-implant head can support multiple forces ormoments that are sufficient to upright or distalizea lingually tipped or rotated molar. This is a big advan-tage in asymmetric movement of individual teeth, sincethe force vector is better controlled. Removal of theC-implant after active treatment is not difficult.

Even though a 4-week healing period is usually re-quired for optimal osseointegration, a C-impant forthese mechanics was used immediately after placement.The first patient had a rapid tipping tendency of thetarget teeth (maxillary right second molar); therefore,we decided to maintain the position of the second molarusing 0.017 3 0.025-in stainless steel wires and a slightopen-coil spring activation through the C-implant hole.The segment wire was applied to the C-implant holepassively, and the initial force was about 50 g. After 1month of activation, the pushing force was increasedto 150 g, and intrusive bending was performed to thesegment archwire. Also, the C-implant in the secondpatient needed to resist a light static load (Fig 12, A)at an initial force after placement.

Also, this patient had a narrow interradicular spacebetween the maxillary right first and second premo-lars. If a mini-implant with 6-mm bone contact is

Fig 12. Treatment progress of the second patient: A, a 0.17 3 0.025-in stainless steel segmentedarchwire was placed that included the maxillary right canine to the right second molar. A C-implant(diameter, 1.8 mm; length, 8.5 mm) was placed between the maxillary right canine and first premolarand used as indirect anchorage for molar distalization. B-D, At a subsequent appointment, direct an-chorage was added with a second segmental .016 3 .022-in stainless steel archwire from theC-implant to the first molar utility arch slot, with the open-coil spring delivering the distal force.E, The remaining full maxillary arch bonding was done after a Class I molar relationship was established.F, All the bonded appliances were removed after distalization. G, Panoramic radiograph 6 months afterimmediate force application. H, Panoramic radiograph 11 months after force application.


placed at 45� to the dental axis, the distance can becalculated as 6 mm of actual length of the screwpart multiplied by Cos 45� 5 4.24 mm.26 A littlemore angulation can be another choice if the mini-implant will be placed in a narrow interradicularspace such as this patient had.

It is simple to activate the coil spring against thetarget tooth by using a crimpable surgical hook. Theclinical tip to prevent the segmented wire from slipping

out of the C-implant during tooth distalization is tosqueeze the extended helix of the segmented archwirein the front of the head part (Fig 12, B and D).

These 2 patients had a slight bite-opening tendencyduring distalization, even though an intrusive forcewas applied to the segmented archwire from theC-implant, probably because of the slight extrusiontendency of the distalized teeth. Intrusive step bendswere made to the segmented archwire during the

Fig 13. Treatment progress occlusal photographs of the second patient: 0.9 mm diameter stainlesssteel bonded transpalatal archwire was applied during asymmetric teeth distalization.

Fig 14. Posttreatment intraoral photographs and panoramic radiograph of the second patient.


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finishing stage. We should have applied the intrusionbends earlier in these patients. However, the healingperiods were insufficient for secondary stability ofthe mini-implant in the first patient because of therapid tipping tendency of the maxillary second molar.Also, a ligature wire to the mini-implant in the secondpatient was insufficient for indirect anchorage(Fig 12, B and C).

In patients who need torque control of the targetteeth, the clinician can fix a rectangular wire segmentinto the hole in the implant head with composite resin.The partially osseointegrated C-implant will supportthis kind of force application.

These clinical reports illustrate the use of the C-implant, with minimal orthodontic attachments and coil-spring biomechanics to distalize tipped and rotated molars.


REFERENCES

1. Armstrong MM. Controlling the magnitude, direction, and dura-

tion of extraoral force. Am J Orthod 1971;59:217-43.

2. Brosh T, Portal S, Sarne O, Vardimon AD. Unequal outer and

inner bow configurations: comparing 2 asymmetric headgear

systems. Am J Orthod Dentofacial Orthop 2005;128:68-77.

3. Wohl T, Bamonte E, Pearson H. Nonextraction treatment of uni-

lateral Class II Division 1 malocclusion with asymmetric head-

gear. Am J Orthod Dentofacial Orthop 1998;113:483-7.

4. Maino BG, Alessandrini P, Mura P. A modified ACCO for Class II

nonextraction treatment. J Clin Orthod 2006;40:605-12.

5. Gumus A, Arat Z. A removable Class II appliance for simulta-

neous distalization and expansion. J Clin Orthod 2005;39:613-7.

6. Jasper JJ, McNamara J. The correction of interarch malocclusions

using a fixed force module. Am J Orthod Dentofacial Orthop

1995;108:641-50.

7. Weiland FJ, Bantleon HP. Treatment of Class II malocclusions

with Jasper jumper appliance—a preliminary report. Am J Orthod

Dentofacial Orthop 1995;108:341-50.

8. Bussick T, McNamara J. Dentoalveolar and skeletal changes asso-

ciated with the pendulum appliance. Am J Orthod Dentofacial

Orthop 2000;117:333-43.

9. Chiu P, McNamara J, Franchi L. A comparisonof two intraoral

molar distalization appliances: distal jet versus pendulum. Am J

Orthod Dentofacial Orthop 2005;128:353-65.

10. Ngantung V, Nanda R, Bowman S. Posttreatment evaluation of the

distal jet appliance. Am J Orthod Dentofacial Orthop 2001;120:

178-85.

11. Carano A, Testa M. The distal jet for upper molar distalization.

J Clin Orthod 1996;30:374-80.

12. Kinzinger G, Diedrich P, Bowman S. Upper molar distalization

with a miniscrew supported distal jet. J Clin Orthod 2006;40:

672-8.

13. Escobar S, Tellez P, Moncada C, Villegas C, Latorre C, Oberti G.

Distalization of maxillary molars with the bone-supported pendu-

lum: a clinical study. Am J Orthod Dentofacial Orthop 2007;131:

545-9.

14. Oncay G, Seckin O, Dincer B, Arikan F. Osseointegrated implants

with pendulum springs for maxillary molar distalization: a cepha-

lometric study. Am J Orthod Dentofacial Orthop 2007;131:16-26.

15. Oncay G, Akyalcin S, Arikan F. The effectiveness of a single os-

seointegrated implant combined with pendulum springs for molar

distalization. Am J Orthod Dentofacial Orthop 2007;131:277-84.

16. Sugawara J, Kanzaki R, Takahashi I, Nagasaka H, Nanda R. Distal

movement of maxillary molars in nongrowing patients with the

skeletal anchorage system. Am J Orthod Dentofacial Orthop

2006;129:723-33.

17. Park HS. Nonextraction treatment with microscrew implants.

Angle Orthod 2004;74:539-49.

18. Chung KR, Kim SH, Kook YA. The C-orthodontic micro-implant.

J Clin Orthod 2004;38:478-85.

19. Oh NH, Kim SH, Kook YA, Mo SS. Study on removal torque of

SLA (sandblasted, large grit and acid etched) treated microim-

plant. Korean J Orthod 2006;36:324-30.

20. Ko TS, Ji YJ, Kim SH, Kook YA, Gong HG, Song HC. The com-

parison of removal torque values and SEM findings of orthodontic

C-implant before and after recycling procedure. J Korean Assoc

Hosp Dent 2006;2:88-95.

21. Chung KR, Nelson G, Kim SH, Kook YA. Severe bidentoalveolar

protrusion treated with orthodontic microimplant-dependent en-

masse retraction. Am J Orthod Dentofacial Orthop 2007;132:

105-15.

22. Seo WK, Kim SH, Chung KR, Nelson G. A pilot study of the

osseointegration potential of a surface-treated mini-implant:

bone-implant contact (BIC) of clinically retrieved C-implants

from human patients. World J Orthod 2009;10:202-10.

23. Chung KR, Kim SH, Kook YA. Joo-Hyun Sohn. Anterior torque

control using partial osseointegration-based mini-implants: bio-

creative therapy type I technique. World J Orthod 2008;9:95-104.

24. Chung KR, Kim SH, Kook YA, Choo H. Anterior torque control

using partial-osseointegration-based mini-implants: biocreative

therapy type II technique. World J Orthod 2008;9:105-13.

25. Kim SH, Cho JH, Chung KR, Kook YA, Nelson G. Evaluation of

the removal torque values of surface-treated mini-implants after

loading. Am J Orthod Dentofacial Orthop 2008;134:36-43.

26. Kim SH, Yoon HG, Choi YS, Hwang EH, Kook YA, Nelson G.

Evaluation of interdental space of the maxillary posterior area

for orthodontic mini-implants with cone-beam computed tomog-

raphy. Am J Orthod Dentofacial Orthop 2009;135:635-41.

27. Park HS, Kyung HM, Sung JH. A simple method of molar upright-

ing with micro-implant anchorage. J Clin Orthod 2002;36:592-6.

28. Lee KJ, Park YC, Hwang WS, Seong EH. Uprighting mandibular

second molars with direct miniscrew anchorage. J Clin Orthod

2007;41:627-35.

29. Derton N, Perini A, Derton R, Blondi G. Orthodontic displace-

ment of mandibular third molars using the orthodontic anchorage

spider screw (OASS) system. Int Orthod 2007;5:129-41.

30. Gracco A, Lombardo L, Cozzani M, Siciliani G. Uprighting

mesially inclined mandibular second molars with a modified

uprighter jet. J Clin Orthod 2007;41:281-4.

31. Kim SH, Kook YA, Jeong DM, Lee W, Chung KR, Nelson G.

Clinical application of accelerated osteogenic orthodontics and

partially osseointegrated mini-implants for minor tooth move-

ment. Am J Orthod Dentofacial Orthop 2009;136:431-9.

Documents

Molar distalization with a partially integrated mini-implant to … · 2017-12-04 · Molar distalization with a partially integrated mini-implant to correct unilateral Class II malocclusion